The present invention is directed to a method of improving adhesion between metal and polymeric materials. More specifically, the present invention is directed to a method of improving adhesion between metal and polymeric materials by treating the metal with an epoxy resin following an adhesion promotion step.
Printed circuits containing one or more circuitry innerlayers are in prominent use today as demand increases for further and further weight and space conservation in electronic devices.
In a fabrication of a multilayer printed circuit, patterned circuitry innerlayers are first prepared by a process in which a copper foil-clad dielectric substrate material is patterned with resist in the positive image of the desired circuitry pattern, followed by etching away of the exposed copper. Upon removal of the resist, there remains the desired copper circuitry pattern. One or more circuitry innerlayers of any particular type or types of circuitry pattern, as well as circuitry innerlayers which might constitute ground planes and power planes, are assembled into a multilayer circuitry by interposing one or more partially-cured dielectric substrate material layers (so called xe2x80x9cpre-pregxe2x80x9d layers) between the circuitry innerlayers to form a composite of an alternating circuitry innerlayers and dielectric substrate material. The composite is then subjected to heat and pressure to cure the partially cured substrate material and achieve bonding of circuitry innerlayers thereto. The cured composite will then have a number of through holes drilled therethrough, which are then metallized to provide a means for conductively interconnecting all circuitry layers. In the course of the through hole metallizing process, desired circuitry patterns will also typically be formed in the outer facing layers of the multilayer composite.
An alternate approach to the formation of a multilayer printed circuit board is through additive or surface laminer circuitry techniques. These techniques begin with a non-conductive substrate, upon which the circuit elements are additively plated. Further layers are achieved by repeatedly applying an imageable coating upon the circuitry and plating further circuit elements upon the imageable coating.
It has long been known that the strength of the adhesive bond formed between the copper metal of the circuitry innerlayers and the cured pre-preg layers, or other non-conductive coatings, in contact therewith leaves something to be desired, with the result that the cured multilayer composite or the coating is susceptible to delamination in subsequent processing and/or use. In response to this problem, the art developed the technique of forming on the copper surfaces of the circuitry innerlayers (before assembling them with pre-preg layers into a multilayer composite) a layer of copper oxide, such as by chemical oxidation of the copper surfaces. The earliest efforts in this regard (so-called xe2x80x9cblack oxidexe2x80x9d adhesion promoters) produced somewhat minimal improvement in the bonding of the circuitry innerlayers to the dielectric substrate layers in the final multilayer circuit, as compared to that obtained without copper oxide provision. Subsequent variations on the black oxide technique included methods wherein there is first produced a black oxide coating on the copper surface, followed by post-treatment of the black oxide deposit with 15% sulfuric acid to produce a xe2x80x9cred oxidexe2x80x9d to serve as the adhesion promoter, such as disclosed by A. G. Osborne, xe2x80x9cAn Alternate Route To Red Oxide For Inner Layersxe2x80x9d, PC Fab. August, 1984, as well as variations involving direct formation of red oxide adhesion promoter, with varying degrees of success being obtained. The most notable improvement in this art is represented in the U.S. Pat. Nos. 4,409,037 and 4,844,981 to Landau, the teachings both of which are incorporated herein by reference in their entirety, involving oxides formed from relatively high chlorite/relatively low caustic copper oxidizing compositions, and producing substantially improved results in circuitry inner layer adhesion.
As earlier noted, the assembled and cured multilayer circuit composite is provided with through-holes which then require metallization in order to serve as a means for conductive interconnection of the circuitry layers of the circuit. The metallizing of the through-holes involves steps of resin desmearing of the hole surfaces, catalytic activation, electroless copper depositing, electrolytic copper depositing, and the like. Many of these process steps involve the use of media, such as acids, which are capable of dissolving the copper oxide adhesion promoter coating on the circuitry innerlayer portions exposed at or neat the through hole. This localized dissolution of the copper oxide, which is evidenced by formation around the through-hole of a pink ring or halo (owing to the pink color of the underlying copper metal thereby exposed), can in turn lead to localized delamination in the multilayer circuit. The art is well aware of this xe2x80x9cpink ringxe2x80x9d phenomenon, and has expended extensive effort in seeking to arrive at a multilayer printed circuit fabrication process which is not susceptible to such localized delamination. One suggested approach has been to provide the adhesion promoting copper oxide as a thick coating so as to retard its dissolution in subsequent processing simply by virtue of sheer volume of copper oxide present. This turns out to be essentially counter-productive, however, because the thicker oxide coating is inherently less effective as an adhesion promoter per se. Other suggestions relating to optimization of the pressing/curing conditions for assembling the multilayer composite have met with only limited success.
Other approaches to this problem involve post-treatment of the copper oxide adhesion promoter coating prior to assembly of circuitry innerlayers and pre-preg layers into a multilayer composite. For example, U.S. Pat. No. 4,775,444 to Cordani discloses a process in which the copper surfaces of the circuitry innerlayers are first provided with a copper oxide coating and then contacted with an aqueous chromic acid solution before the circuitry innerlayers are incorporated into the multilayer assembly. The treatment serves to stabilize and/or protect the copper oxide coating from dissolution in the acidic media encountered in subsequent processing steps (e.g., through-hole metallization), thereby minimizing pink ring/delamination possibilities.
U.S. Pat. No. 4,642,161 to Akahoshi et al., U.S. Pat. No. 4,902,551 to Nakaso et al., and U.S. Pat. No. 4,981,560 to Kajihara et al., and a number of references cited therein, relate to processes in which the copper surfaces of the circuitry innerlayers, prior to incorporation of the circuitry innerlayers into a multilayer circuit assembly, are first treated to provide a surface coating of adhesion-promoting copper oxide. The copper oxide so formed is then reduced to metallic copper using particular reducing agents and conditions. As a consequence, the multilayer assembly employing such circuitry innerlayers will not evidence pink ring formation since there is no copper oxide present for localized dissolution, and localized exposure of underlying copper, in subsequent through-hole processing. As with other techniques, however, processes of this type are suspect in terms of the adhesion attainable between the dielectric substrate layers and the metallic copper circuitry innerlayers. This is particularly so in these reduction processes since the circuitry bonding surface not only is metallic copper, but also presents the metallic copper in distinct phases (i.e. (1) copper-from-reduction-of-copper oxide over (2) copper of the copper foil) which are prone to separation/delamination along the phase boundary.
U.S. Pat. Nos. 4,997,722 and 4,997,516 to Adler similarly involve formation of a copper oxide coating on the copper surfaces of circuitry innerlayers, followed by treatment with a specialized reducing solution to reduce the copper oxide to metallic copper. Certain portions of the copper oxide apparently may not be reduced all the way to metallic copper (being reduced instead to hydrous cuprous oxide or cuprous hydroxide), and those species are thereafter dissolved away in a non-oxidizing acid which does not attack or dissolve the portions already reduced to metallic copper. As such, the multilayer assembly employing such circuitry innerlayers will not evidence pink ring formation since there is no copper oxide present for localized dissolution, and localized exposure of underlying copper, in subsequent through-hole processing. Here again, however problems can arise in terms of the adhesion between the dielectric layers and metallic copper circuitry innerlayers, firstly because the bonding surface is metallic copper, and secondly because the metallic copper predominately is present in distinct phases (i.e., (1) copper-from-reduction-of-copper oxide over (2) copper of the copper foil), a situation prone to separation/delamination along the phase boundary.
U.S. Pat. No. 5,289,630 to Ferrier et al., the teachings of which are incorporated herein by reference in their entirety, reveals a process whereby an adhesion promotion layer of copper oxide is formed on the circuit elements followed by a controlled dissolution and removal of a substantial amount of the copper oxide in a manner which does not adversely affect the topography.
PCT Application No. WO96/19097 to McGrath, the teachings of which are incorporated by reference herein in their entirety, discusses a process for improving the adhesion of polymeric materials to a metal surface. The process discussed involves contacting the metal surface with an adhesion-promoting composition comprising hydrogen peroxide, an inorganic acid, a corrosion-inhibitor and a quaternary ammonium surfactant.
U.S. Pat. No. 5,869,130, to Ferrier entitled xe2x80x9cProcess For Improving Adhesion of Polymeric Materials to Metal Surfacesxe2x80x9d describes a process for improving the adhesion of polymeric materials to a metal surface, especially copper or copper alloy surfaces in the production of multilayer printed circuits. The process described in this patent application provides for contacting the metal surface with an adhesion-promoting composition comprising (a) an oxidizer; (b) an acid; (c) a corrosion inhibitor; (d) a source of halide ions; and (e) optionally, a water soluble polymer. That process provided excellent adhesion between the metallic and polymeric surfaces (i.e. the circuitry and the intermediate insulating layer), while eliminating or minimizing pink ring and operating economically, as compared to above noted conventional processes.
However, it has been found that the use of acidic peroxide treatment solutions, while giving good peel strength and stability of copper laminated to fiberglass filled resin, allows the occurrence of some flaws on further processing of the laminated board to create a finished printed circuit board. Thus, with some resin systems, particularly lower Tg materials, drilling removes a small quantity of organic material from the surface of the etch treated copper at the point where the drill intersects with the copper layer; this phenomenon is called xe2x80x9cwedgexe2x80x9d. Further processing the drilled board through the chemical steps of preparing and plating the board creates a small area at the treated copper surface around the drilled hole where the coating is attacked and removed, creating a phenomenon called xe2x80x9cpink ringxe2x80x9d. This area of pink ring is much smaller than the pink ring observed in standard oxide processing to prepare copper surfaces for lamination. Nevertheless, in this area of attack small areas of resin contracting from the copper surface can be observed after floating the board in molten solder. Although these areas of resin contraction (called xe2x80x9cresin voidsxe2x80x9d) may not be sufficient cause for rejection of the board they are still a concern.
In particular, it has been found that these wedge, pink ring and resin void flaws can be greatly reduced or eliminated by processing the board treated with these acidic peroxide treatment solutions through a strongly alkaline post treatment, followed by rinsing and drying.
U.S. Pat. No. 6,020,029 to Ferrier et al. discloses an acidic peroxide, adhesion promotion composition containing an oxidizer, an acid, a corrosion inhibitor, optionally a source of halide ions and optionally a water soluble polymer. Preferably, the water-soluble polymer is not a wetting agent or surfactant, but a water soluble homopolymer or copolymer of low molecular weight. The composition is applied to a metal surface. After the acidic peroxide solution is applied to the metal, the metal is post-treated with a strong alkaline solution followed by rinsing with water. After the metal is post-treated, a polymeric material is bonded to the metal surface. The ""029 patent alleges that the combination of the halide and water soluble polymer in the acidic peroxide composition provide the best bonding results between the metal surface and the polymeric material.
WO 00-02426 discloses another method for addressing bond integrity between a metal surface and a polymeric coating. The method disclosed in the WO 00-02426 patent is to convert cupric-based organometallic compounds into cuprous-based organometallic conversion coatings to improve bonding integrity. The process is achieved by reduction and/or by partial dissolution of the organometallic coating, and optionally applying a copper oxidation inhibitor. Reducing agents employed include aminoboranes and dissolution agents include cupric ion chelators, organic or inorganic acids and their salts. Copper oxidation inhibitors include azole derivatives, including halogen substituted derivatives thereof. Cationic, amphoteric, anionic and/or non-ionic surface active agents may be included in the composition to enhance the effectiveness of the composition.
Although there are numerous methods of improving the bonding integrity between a metal surface and a polymeric material, there is still a desire and a need in the circuit board industry to increase adhesion between a metal surface and a polymeric material. Many of the above disclosed methods employ treatment solutions that are acidic or alkaline and cause the formation of a textured surface on the metal. However, none of the above-disclosed methods attempt to exploit the textured metal surface to further enhance bonding between the metal and the polymeric material. Accordingly, there is a need for a process that exploits the textured surface of the metal to improve the adhesive properties between the metal and polymeric material.
The present invention is directed to a process and composition for improving the adhesion between a metal surface and a polymeric material by treating the metal surface with an adhesion promotion composition followed by contacting the treated metal surface with an epoxy resin composition. The epoxy resin composition may be aqueous based, or organic based. The epoxy resin composition makes the metal surface more accessible to contact with a polymeric material that is coated on the metal surface. After the treated metal surface is post-treated with the epoxy resin composition, the polymeric material is placed on the surface of the metal to form a high integrity bond between the metal surface and the polymer material. Advantageously, the method and composition of the present invention provide for improved adhesion between a metal surface and a polymeric material as compared with known adhesion promoting processes. Accordingly, the adhesion between the metal surface and the polymeric material is such that multilayer circuit boards prepared using the method of the present invention may be employed in electronic devices without concern that the polymeric material may delaminate or peel from the metal surface.
Also, the post-treatment method of the present invention may be employed with many different adhesion promotion treatments to further improve adhesion between a metal surface and a polymeric material. Thus the process of the present invention is a readily available means of improving the adhesion promoting methods known in the circuit board industry.
An objective of the present invention is to provide a means of improving the adhesion integrity between a metal surface and a polymeric material.
Another objective is to provide an improved peel strength between a metal surface and a polymeric coating.
An additional objective of the present invention is to provide a multilayer circuit board that can be employed in electrical devices without concern that the circuit board will delaminate.
A further objective of the present invention is to provide a method of improving adhesion between a metal surface and a polymeric material that can be employed toimprove known adhesion methods.
Additional objectives and advantages of the present invention will be apparent to those of skill in the art after reading the following description of the invention and the appended claims.